Blood pump with in-situ attaching motor stators

ABSTRACT

A housing having an interior and an exterior. A pump rotor is configured to be received within the interior of the housing, the pump rotor includes a magnet. A stator having a delivery configuration and an operative configuration is included, the stator in the delivery configuration has a delivery diameter, the stator in the operative configuration being configured to be disposed around the exterior of the housing and to form an assembled pump having a diameter greater than the delivery diameter.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to and claims priority to U.S. ProvisionalPatent Application Ser. No. 62/411,781, filed Oct. 24, 2016, entitledBLOOD PUMP WITH IN-SITU ATTACHING MOTOR STATORS, the entirety of whichis incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

n/a

TECHNICAL FIELD

The present invention relates to mechanical circulatory support devicesand methods of implanting and operating the same.

BACKGROUND

Mechanical circulatory support devices are used to assist the pumpingaction of the heart. A mechanical circulatory support device typicallyincludes a pump having pump housing and a moveable element, mosttypically a rotor, disposed within the pump housing to impel bloodthrough the housing. The pump is implanted within a patient with aninlet of the housing in communication with one portion of the patient'scardiovascular system and an outlet of the housing in communication withanother portion of the cardiovascular system. For example, to assist thepumping action of the left ventricle, the inlet of the pump may be incommunication with the interior of the left ventricle, whereas theoutlet of the pump may be in communication with the aorta. One form ofblood pump includes a tubular pump housing having a rotor coaxial withthe housing. The rotor desirably incorporates a permanent magnet. Astator including a plurality of electrical coils is arranged around theexterior of the pump housing. In operation, electrical power is appliedto the coils in sequence so as to create a rotating magnetic fieldwithin the housing. The rotating field interacts with the magnetincorporated in the rotor so as to spin the rotor about its axis andthus force blood through the pump housing. As disclosed in U.S.Published Patent Application No. 2009/0112312 (“the '312 Publication”),the disclosure of which is hereby incorporated by reference herein, itis been proposed to implant the pump within the aorta of a livingsubject. Pumps of this general type typically are made as permanentassemblies, with the elements of the stator fixed in position on theoutside of the pump housing during manufacture and with the entireassembly enclosed within a sealed outer housing.

SUMMARY

The present invention advantageously provides a method and system forattaching motor stators in situ in blood pumps. The blood pump includesa housing having an interior and an exterior. A pump rotor is configuredto be received within the interior of the housing, the pump rotorincludes a magnet. A stator having a delivery configuration and anoperative configuration is included, the stator in the deliveryconfiguration has a delivery diameter, the stator in the operativeconfiguration being configured to be disposed around the exterior of thehousing and to form an assembled pump having a diameter greater than thedelivery diameter.

In another aspect of this embodiment, the housing defines a housing axisand axial directions along the housing axis, the stator including aplurality of units, each unit including a coil having a coil axis and anelectrical conductor arranged in one or more turns surrounding the coilaxis, the stator in the operative configuration having the unitsdisposed around the housing axis with the coil axes of the unitstransverse to the housing axis.

In another aspect of this embodiment, in the delivery configuration: thestator has a delivery axis, the coil axes are transverse to the deliveryaxis, and at least one of the units is offset from another one of theunits in a direction along the delivery axis.

In another aspect of this embodiment, in the delivery configuration, theunits of the stator are disposed in a row along the delivery axis, witheach of the units being offset from an adjacent one of the units in adirection along the delivery axis.

In another aspect of this embodiment, the housing has an exteriorsurface including one or more arcuate surfaces coaxial with the housingaxis, and wherein each of the units has an interior surface in the formof a partial surface of revolution around a unit axis transverse to thecoil axis of that unit, the interior surfaces of each units confrontingthe arcuate surfaces of the housing when the stator is in the operativeconfiguration.

In another aspect of this embodiment, the stator includes a flexiblemember including one or more coils, the flexible member extending aroundthe housing when the stator is in the operative condition.

In another aspect of this embodiment, when the stator is in theoperative configuration, the flexible member forms a tube and the pumphousing is disposed within the tube, and wherein the flexible member isfolded when the stator is in the delivery configuration.

In another embodiment, a blood pump implantation assembly includes adelivery device. A pump is mounted to the delivery device proximate adistal end thereof, the pump including a pump housing, a rotorconfigured to be disposed within the pump housing, and a stator. Thepump in a delivery condition has the stator offset from the pump housingand has the stator define a first diameter. The delivery device isconfigured to transition the pump from the delivery condition to anoperative condition in which the stator is in an operative configurationdisposed around the pump housing and defines a second diameter largerthan the first diameter.

In another aspect of this embodiment, the housing has a housing axisextending proximally and distally when the pump is in the deliverycondition, the stator including a plurality of units, each unitincluding a coil having a coil axis and an electrical conductor arrangedin one or more turns surrounding the coil axis, the pump in theoperative configuration having the units disposed around the housingaxis with the coil axes of the units transverse to the housing axis.

In another aspect of this embodiment, in the delivery configuration thecoil axes are transverse to a major longitudinal axis of the deliverydevice and at least one of the units is offset from another one of theunits along the major longitudinal axis.

In another aspect of this embodiment, in the delivery configuration theunits of the stator are arranged successively along the majorlongitudinal axis.

In another aspect of this embodiment, the delivery device is configuredto advance the units along the major longitudinal axis toward the pumphousing to bring the pump from the delivery condition to the operativecondition.

In another aspect of this embodiment, the stator includes a flexiblemember including one or more coils, the flexible member extending aroundthe housing when the pump is in the operative condition.

In another aspect of this embodiment, the flexible member is a tubehaving a wall, the wall defining a bore and the housing being disposedwithin the bore when the pump is in the operative condition, theflexible member being folded along one or more fold axes when the pumpis in the delivery configuration.

In another aspect of this embodiment, the pump is releasably mounted tothe delivery device.

In yet another embodiment, a method of implanting a blood pump in thebody of a living subject includes advancing the pump into the body ofthe subject to a target location, the pump being in a delivery conditionin which a stator of the pump is separate from a pump housing and thepump has a delivery diameter. The pump is transitioned to an operativecondition, the stator being disposed around the pump housing and thepump has an operative diameter greater than the delivery diameter whenin the operative condition.

In another aspect of this embodiment, the method further includesapplying electrical power to the stator to provide a rotating magneticfield within the housing while the pump is in the operative condition arotor within the housing is rotated and the rotor impels blood throughthe housing.

In another aspect of this embodiment, advancing the pump into the bodyof the subject to a target location step includes advancing the pumpthrough vasculature of the subject.

In another aspect of this embodiment, transitioning the pump to anoperative condition includes at least one from the group consisting ofretracting and advancing a portion of a delivery device, the deliverydevice including the stator in the delivery condition.

In another aspect of this embodiment, the delivery device defines amajor longitudinal axis, and wherein the stator includes a plurality ofstator units disposed successively around the major longitudinal axis.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, and theattendant advantages and features thereof, will be more readilyunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is a diagrammatic perspective view depicting elements of a pumpand delivery device in accordance with one embodiment of the invention;

FIG. 2 is a diagrammatic perspective view depicting one element of thepump of FIG. 1, with certain portions depicted as transparent forclarity of illustration;

FIG. 3 is a further diagrammatic perspective view depicting elements ofthe pump and delivery device of FIG. 1 in one condition;

FIG. 4 is a view similar to FIG. 3, but depicting the same elements ofthe pump in another condition;

FIG. 5 is a diagrammatic elevational view depicting elements of a pumpaccording to a further embodiment of the invention in one condition;

FIG. 6 is a view similar to FIG. 5 depicting elements of FIG. 5 inanother condition;

FIG. 7 is a diagrammatic perspective view depicting an element of a pumpaccording to yet another embodiment of the invention in one condition;and

FIG. 8 is a view similar to FIG. 7 and depicting the same element inanother condition.

DETAILED DESCRIPTION

A pump according one embodiment of the invention includes a tubular pumphousing 10 (FIG. 1) having an inlet end 12 with an inlet opening and anoutlet end 14 with an outlet opening. The pump housing has an interiorsurface defining a housing axis 16 extending between the inlet andoutlet ends. The pump housing 10 also has an exterior surface generallyin the form of a surface of revolution around axis 16. For clarity ofillustration, pump housing 10 is depicted as transparent in FIG. 1. Inpractice, the pump housing 10 typically is formed from a ceramicmaterial or other non-magnetic, biocompatible material. A rotor 18 isdisposed within the interior of the pump housing. In one configuration,the rotor 18 has an axis coincident with the housing axis 16. The rotor18 has a plurality of generally helical blades 20 defining channels 22,one of which is visible in FIG. 1. The channels 22 extend incircumferential directions around the axis of the rotor 18 and extendaxially from an upstream end of the rotor 18 adjacent the inlet end 12of the housing to the downstream end of the rotor 18. The rotor 18incorporates a permanent magnet. For example, the rotor 18 may be formedfrom a single piece of a biocompatible magnetic alloy as, for example, aplatinum cobalt alloy. The outer or tip surfaces of the rotor 18adjacent the wall of the pump housing desirably define hydrodynamicbearing surfaces capable of suspending the rotor out of contact with theinterior wall of the pump housing when the rotor spins about its axisduring operation. One design for such a rotor is set forth in theaforementioned '314 Publication. Other rotor designs are disclosed inU.S. Published Patent Application No. 2015/0051438, the disclosure ofwhich is also incorporated by reference herein.

A flow straightener 24 is disposed within the pump housing 10 downstreamfrom rotor 20. The flow straightener is fixed to the pump housing 10.The flow straightener 24 has vanes arranged to convert rotation momentumaround axis 10 imparted by rotor 20 during operation into momentum inthe downstream direction and useful pressure. For example, the flowstraightener 24 may have helical vanes having a pitch or direction oftwist around axis 16 opposite to the pitch of the blades 20 of therotor.

The pump further includes a stator 30 depicted in a deliveryconfiguration at 30 (to the right in FIG. 1) and in an operativeconfiguration at 30′ (to the left in FIG. 1). Stator 30 includes aplurality of coil units 32, in this case, three such units 32 a, 32 b,and 32 c. An individual one of the coil units 32 is depicted in FIG. 2.Each unit 32 of the stator 30 includes a housing 34 having an arcuateinner surface 36 generally in the form of a partial surface ofrevolution around a unit axis 38. In the embodiments shown, each arcuateinner surface is a sector of a cylinder coaxial with an axis 38. Thearcuate inner surface 36 has a radius R_(is) substantially equal to theradius of the outer surface of pump housing 10. Stated another way, thearcuate inner surface 36 is a surface complimentary to a portion of theouter surface of the pump housing 10.

The housing 34 of each unit 32 includes a tubular guide channel 40extending parallel to the unit axis 38 on the exterior side of thehousing, i.e., on the side opposite from arcuate surface 36. Aferromagnetic pole piece 42 is disposed within the housing 34 of eachunit 32. The pole piece is a generally rectangular solid having acentral axis 48 transverse to the unit axis. The pole piece 42 alsoincludes a wide portion 50 adjacent the inner surface 36. This portiondesirably has an interior surface conforming to the interior surface 36of the housing. The wide portion 50 of the pole piece may define aportion of the interior surface.

A coil 52 includes a plurality of turns 54 of an electrical conductorsuch as a wire extending around pole piece 42 and axis 48, only one suchturn being shown in broken lines in FIG. 2 for clarity of illustration.External conductive leads (not shown) are electrically connected to coil52. These external leads extend from the housing. The housing protectsthe coil from the surrounding environment.

A set of elongated, flexible guides 56 a-56 c are fastened to the pumphousing 10 (FIG. 3). Each guide has a fixed end 58 attached to the pumphousing adjacent the inlet end 12 of the housing. The fixed ends 58 ofthe guides are fastened to the exterior surface of the pump housing atequally spaced intervals around the axis of the pump housing. Each guideextends through the guide channel 40 (FIG. 2) of one stator unit 32.Thus, guide 56 a extends through the channel of stator unit 32 a (FIG.3), whereas guide 56 b extends through the channel of stator unit 32 b,and guide 56 c extends through the channel of stator unit 32 c.

The external surface of pump housing 10 may have additional guideelements such as small ribs projecting from its outer surface (notshown) and extending generally in the axial direction. These ribs may bearranged to engage the housings of stator units 32 a-32 b and to guideeach such stator unit into the operative position 30′ of the statorshown in FIGS. 1-4. Also, the exterior of the pump housing 10 may beprovided with one or more stops for arresting motion of each stator unitalong the axial direction of the pump housing when the stator unitreaches the operative position as discussed below. A flexible, hollowinlet tube 60 (FIG. 3) is connected to the inlet end 12 of pump housing10. The tube 60 has a smooth inlet end 62 with opening 64.

The pump can be implanted by a delivery device 70. The delivery device70 includes a central shaft 72 (FIG. 1) defining a major longitudinalaxis extending in a distal direction denoted by arrow D in FIG. 1 and anopposite proximal direction denoted by arrow P. Only the distal end ofthe shaft is visible in FIG. 1. A fitting 74 at the distal end of shaft72 defines a conical guide surface 76 flaring outwardly in the distaldirection. This guide surface is concentric with central shaft 72.Fitting 74 further defines a seat 78 adapted to engage the outlet end 14of housing 10 and thus hold the housing on the central shaft with theaxis 16 of the housing coaxial with the distal end of shaft 72. The seat78 desirably is adapted to release from the housing upon command. Forexample, the housing 10 may have features such as one or more holes orprojections (not shown) on its interior or exterior surfaces, and seat78 may have one or more movable elements such as pins or hooks adaptedto engage these features of the housing 10. These elements desirably canbe selectively actuated by an operator from the proximal end of theshaft. For example, the shaft 72 may include one or more flexible wires(not shown) extending within it so that movement can be transmitted fromthe proximal end of the shaft to the hook or pin to release housing 10from its engagement with seat 78. Any other device capable of actuatingan element at a distal end of a shaft can be used. For example, elementsdriven by fluid pressure, such as balloons, may be used to actuate therelease elements of the seat. In other embodiments, housing 10 may beheld to seat 78 by one or more sutures that extend through the centralshaft 72 or through other elements of the delivery device 70, and thehousing may be released from the shaft by pulling on an end of thesuture exposed at the proximal end of the shaft. Shaft 72 and fitting 74are omitted in FIGS. 3 and 4 for clarity of illustration.

In the delivery configuration shown in FIGS. 1 and 3, the units 32 a, 32b, and 32 c of stator 30 are arranged in a row, i.e. successively, andextend along central shaft 72 and a delivery axis substantially coaxialwith the axis of the housing 10. Each unit 32 is offset from the nextadjacent unit in the proximal direction indicated by arrow P or in theopposite, distal direction D along the delivery axis. Stated anotherway, in this embodiment, only one unit 32 is present at any point alongthe proximal or distal extent of the central shaft 72 and along thedelivery axis. Thus, the entire stator 30, in its deliveryconfiguration, has a diameter in a plane transverse to the delivery axisapproximately equal to the width of a single stator unit 32 (FIG. 2). Asused in this disclose with reference to a structure having anon-circular cross-sectional shape, the term “diameter” refers to thediameter of the smallest circle in a plane perpendicular to the axis ofthe structure through which the structure can pass.

The delivery device 70 is also configured to transition the stator units32 so as to bring the stator 30 from the delivery configuration to theoperative configuration illustrated at 30′ in FIG. 1 and illustrated inFIG. 4. One structure that is capable of performing this function isschematically indicated in FIG. 3. This structure includes a pluralityof hollow pusher shafts 80 disposed within an outer sheath 82. Each suchpusher shaft 80 surrounds one of the guides 56 a-56 c. The central shaft72 (FIG. 1) is also disposed within outer shaft 82, but is omitted inFIG. 3 for clarity of illustration. A proximal end of each pusher shaft80 is accessible at the proximal end of the outer shaft 82. Only thedistal end of shaft 82 is shown in FIG. 3. While the stator units 32 arein the delivery configuration, the outer shaft 82 may surround thestator units 32 to retain the stator units 32 in position relative tothe inner shaft 72. The outer shaft 82 may be retracted proximallyrelative to the inner shaft 72 and stator units 32 to the positiondepicted in FIG. 3 to expose the stator units 32. While the outer shaft82 is so retracted, each pusher shaft 80 can be slid in the distaldirection D along the associated guide so as to force one of the statorunits 32 from its delivery position to its position in the operativeconfiguration. For example, shaft 80 a can be slid along guide 56 a tomove stator unit 32 a distally. As the stator unit 32 moves distally, itslides over the flaring conical surface 76 of fitting 74 and slides overthe outer surface of pump housing 10 to its operative position indicatedat 32 a′ in FIG. 1. The other units 32 b and 32 c can be positioned intheir respective final positions by moving the other shafts 80 b and 80c in a similar manner. This movement device is merely exemplary. Othermechanisms capable of moving elements at or near the distal end of adelivery device 70 while the distal end is disposed within the body of asubject may be used. For example, resilient elements, shape memoryalloys, electrically-actuated mechanisms, and fluid powered elementssuch as balloons. In a further variant, suture loops may be used topull, rather than push, the stator units.

When the pump is in its delivery condition, with the stator in thedelivery configuration, the pump as a whole has a relatively smalldiameter. Stated another way, the entire pump can pass through a circlehaving a diameter equal to or slightly larger than the diameter of pumphousing 10 alone. By contrast, when the pump is in an operativecondition, with the stator in the operative configuration illustrated at30′ in FIG. 1 and shown in FIG. 4, the diameter of the pump is largerthan the diameter of the pump housing.

In a method according to a further aspect of the present disclosure, thepump is implanted into a body of a living subject. While the pump is inits delivery condition, with the pump housing attached to fitting 74 ofthe central shaft and with the stator in its delivery configuration, thedelivery device 70 and pump are advanced into the body of a subject.Typically, the pump is to be implanted in the vascular system and thusis advanced through the vasculature of the subject. For example, thepump may be inserted into a peripheral artery of the subject andadvanced towards a target location. Typically, the target location is inthe ascending or descending aorta. In one such procedure the pump may beintroduced through an opening in the femoral artery through a smallincision in the skin. In other cases, the pump may be introduced into anartery through an aortotomy in an artery remote from the aorta and againadvanced through the arteries toward the aorta until the pump reaches atarget location. When the pump is position in the ascending aorta, theinlet tube 60 (FIGS. 3-4) may extend through the aortic valve into theleft ventricle of the heart. Even where the particular dimensions of thesubject or the target location for the pump preclude advancement throughthe vasculature, the small delivery diameter facilitates advancement ofthe pump into the subject's body. For example, the pump may be placedthrough a surgically created opening in the patient's heart and advancedthrough the heart or through another organ to the target location.

Once the pump is at or near the target location, the delivery device 70is actuated to bring the stator to its operative configuration and thusbring the pump to the operative condition. For example, outer sheath 82may be retracted to expose the stator units, and pusher tubes 80 may beadvanced distally to move the stator units 32 to the operative position30′ shown in FIG. 4. Fitting 74 (FIG. 1) on the inner shaft 72 isdisengaged from the housing 10 of the pump, and the delivery device 70is removed. The leads (not shown) associated with the stator units maybe left in place, extending through the vasculature to the locationwhere the delivery device 70 was inserted into the vasculature, and maybe connected to a conventional controller (not shown) adapted to providepower to the stator units. The guides 56 a-56 c may remain in place ormay be removed. With the stator units the operative position, in placearound the pump housing, electrical power is applied to the coils in thenormal manner, through the leads associated with the stator units. Theelectrical current within the coils of the stator units creates arotating magnetic field turning around axis 16 of the pump housing. Thisrotating field drives the rotor and causes the rotor to impel bloodthrough the pump, from inlet cannula 62 to the outlet of the pumphousing.

The pump optionally may include a device such as an expandable anchor(not shown) attached to the pump housing 10 or to one or more of thestator units. As described, for example, in the aforementioned '312Publication, such an expandable anchor may hold the pump in location as,for example, at a target location in the aorta. Desirably, theexpandable anchor is in a collapsed condition when the pump is advancedinto the vasculature.

Numerous variations and combinations of the features discussed above canbe used. For example, a pump according to a further embodiment of theinvention has a stator with a delivery configuration shown in FIG. 5 andan operative configuration shown in FIG. 6. The stator 130 includesplural stator units 152 similar to the stator units discussed above. Inthe delivery configuration 130 (FIG. 5), the stator units are notarranged in a row, but rather are stacked on one another. The statorunits are connected to one another by one or more elements 101 formedfrom deformable material. Each such element may be a web or mesh of ashape memory alloy as, for example, a nitinol alloy. Each such element101 is arranged to deform spontaneously from its shape as shown in thedelivery configuration to an operative shape 101′ (FIG. 6). For example,the shape memory alloy may be arranged to deform upon exposure to bodytemperature. Alternatively, the shape memory alloy may be arranged todeform to the operative position under its own elasticity and may berestrained in the delivery configuration by the delivery device 70. Thestator in its operative condition 130′ forms a generally tubular unitwith an interior space between the stator units. In the operativeconfiguration, the stator may be slid onto the outside of the pumphousing, so that the pump housing is received within this generallytubular unit.

In a further variant (FIGS. 7-8), a stator in its operative conditionincludes a generally tubular housing having a wall 235 defining aninterior bore 233. Coils 254, one of which is schematically depicted inFIG. 7, are embedded within the wall 235. In the delivery configuration,the stator is compressed by folding wall 235 generally along linesparallel to the axis of the tubular housing so as to bring one portion235 a of the wall close to another portion 235 b, within the spacenormally occupied by bore 233, and thus compress the stator to a smallerdiameter. The stator according to this embodiment may omit theferromagnetic cores usually present in the stator coils. The stator 230may be brought to the operative configuration 230′ by the resilience ofthe wall. For example, the stator may be constrained by a sheathincorporated in the delivery device 70 and may revert to the operativeconfiguration when the stator is ejected from the sheath. Alternatively,a mechanical expansion device or an inflatable device such as a balloonmay be disposed within the stator. The balloon is inflated to bring thestator to its operative condition.

Where ferromagnetic cores are provided in the stator (as in theembodiment discussed above with reference to FIGS. 1-4), magneticattraction between the poles of the magnet incorporated in the rotor andthe ferromagnetic cores can maintain the axial position of the rotorwith respect to the housing. If the ferromagnetic cores are omitted, thehousing may incorporate ferromagnetic elements to perform a similarfunction. Alternatively or additionally, the rotor may be provided withhydrodynamic thrust bearings and the housing may be provided withcomplementary surfaces so that hydrodynamic action maintains the rotorin position relative to the housing. In still other embodiments, thehousing and rotor may incorporate mechanical bearings to or to controlthe axial position of the rotor relative to the housing, to maintain therotor coaxial with the stator, or both.

As these and other variations and combinations of the features describedhereinabove may be used, the foregoing description should be taken byway of illustration rather than by way of limitation of the presentdisclosure.

What is claimed is:
 1. A blood pump, comprising: a housing having an interior and an exterior, the housing defining a housing axis and an axial direction along the housing axis; a pump rotor configured to be received within the interior of the housing, the pump rotor including a magnet; and a stator having a plurality of stator units, each stator unit having a coil and a delivery configuration and an operative configuration, the plurality of stator units in the delivery configuration having a delivery diameter, each stator unit being configured to slide successively along a common shaft, the plurality of stators units in the operative configuration being configured to: be disposed around the exterior of the housing; and form an assembled pump having a diameter greater than the delivery diameter.
 2. The blood pump of claim 1, wherein the housing defines a housing axis and axial directions along the housing axis, each coil within each unit of the plurality of stator units including a coil axis and an electrical conductor arranged in one or more turns surrounding the coil axis, the stator in the operative configuration having the units disposed around the housing axis with the coil axes of the units transverse to the housing axis.
 3. The blood pump of claim 2, wherein in the delivery configuration: the stator has a delivery axis; the coil axes are transverse to the delivery axis; and at least one of the units is offset from another one of the units in a direction along the delivery axis.
 4. The blood pump of claim 3, wherein in the delivery configuration the units of the stator are disposed in a row along the delivery axis, with each of the units being offset from an adjacent one of the units in a direction along the delivery axis.
 5. The blood pump of claim 3, wherein the housing has an exterior surface including one or more arcuate surfaces coaxial with the housing axis, and wherein each of the units has an interior surface in the form of a partial surface of revolution around a unit axis transverse to the coil axis of that unit, the interior surfaces of each units confronting the arcuate surfaces of the housing when the stator is in the operative configuration.
 6. The blood pump of claim 1, wherein the stator includes a flexible member including one or more coils, the flexible member extending around the housing when the stator is in the operative condition.
 7. The blood pump of claim 6, wherein when the stator is in the operative configuration, the flexible member forms a tube and the pump housing is disposed within the tube, and wherein the flexible member is folded when the stator is in the delivery configuration.
 8. A blood pump implantation assembly, comprising: a delivery device; a pump mounted to the delivery device proximate a distal end thereof, the pump including a pump housing, a rotor configured to be disposed within the pump housing and a stator having stator units, the pump in the operative configuration having the units disposed around the housing axis with the coil axes of the units transverse to the housing axis; the pump in a delivery condition having the stator units offset from the pump housing and having the stator define a first diameter, the delivery device being configured to transition the pump from the delivery condition to an operative condition in which the stator is in an operative configuration disposed around the pump housing and defines a second diameter larger than the first diameter, each stator unit being configured to slide successively along a common shaft in the delivery condition.
 9. The assembly of claim 8, wherein the housing has a housing axis extending proximally and distally when the pump is in the delivery condition, the stator including a plurality of units, each unit including a coil having a coil axis and an electrical conductor arranged in one or more turns surrounding the coil axis.
 10. The assembly of claim 9, wherein in the delivery configuration the coil axes are transverse to a major longitudinal axis of the delivery device and at least one of the units is offset from another one of the units along the major longitudinal axis.
 11. The assembly of claim 10, wherein in the delivery configuration the units of the stator are arranged successively along the major longitudinal axis.
 12. The assembly of claim 11, wherein the delivery device is configured to advance the units along the major longitudinal axis toward the pump housing to bring the pump from the delivery condition to the operative condition.
 13. The assembly of claim 8, wherein the stator includes a flexible member including one or more coils, the flexible member extending around the housing when the pump is in the operative condition.
 14. The assembly of claim 13, wherein the flexible member is a tube having a wall, the wall defining a bore and the housing being disposed within the bore when the pump is in the operative condition, the flexible member being folded along one or more fold axes when the pump is in the delivery configuration.
 15. The assembly of claim 8, wherein the pump is releasably mounted to the delivery device.
 16. A method of implanting a blood pump in the body of a living subject, comprising: advancing the pump into the body of the subject to a target location, the pump being in a delivery condition in which a stator of the pump having stator units is separate from a pump housing and the pump has a delivery diameter; and sliding the stator units successively along a common shaft; and transitioning the pump to an operative condition, the stator being disposed around the pump housing and the pump has an operative diameter greater than the delivery diameter when in the operative condition.
 17. The method of claim 16, further comprising applying electrical power to the stator to provide a rotating magnetic field within the housing while the pump is in the operative condition a rotor within the housing is rotated and the rotor impels blood through the housing.
 18. The method of claim 17, wherein advancing the pump into the body of the subject to a target location step includes advancing the pump through vasculature of the subject.
 19. The method of claim 16, wherein transitioning the pump to an operative condition includes at least one from the group consisting of retracting and advancing a portion of a delivery device, the delivery device including the stator in the delivery condition. 